This invention relates generally to a duplex analog voice-band scrambler for secure communications and more particularly to a multiple hop frequency inversion scrambling device for limited bandwidth communications channels such as standard telephone lines and radiotelephone communication circuits.
Communications between individuals on an unsecure communications channel are well known to be subject to casual eavesdropping or more malicious interception of messages. Conventional wireline communications, i.e. telephone calls, while protected by law, are still susceptible to illegal wiretapping and interception of messages but with some difficulty. The problem becomes even more severe when the communications channel utilizes radio links to convey the messages. Lawful means of receiving radio channels exist and provide easy access to the messages being carried via radio. Cellular radiotelephone systems offer a particularly severe combination of technology and mental state of the typical user which provides easy access to messages carried by the systems. The communications channel in a cellular radiotelephone system generally consists of both radio and landline links, each link being available to its own type of message interception. Furthermore, the typical cellular radiotelephone user thinks of the radiotelephone as an extension of the landline system (as it is) and therefore not particularly easy to intercept messages. Unfortunately, this is not the case.
To protect the security of messages transmitted over a communications channel, two broad categories of security-creating have been devised. Analog messages, such as voice, may be converted to digital signal representations of the analog signal or textual material may be represented by a digital signal. The digital signal may then be permuted into a cryptographic signal by arithmetric processes using secret or public encyihering keys and subsequently transmitted over an unsecure channel. The intended recipient of the message can receive the cryptographic signal, decipher the signal using a secret deciphering key, and recover the message. Further background for this technique may be found in "The Mathematics of Public-Key Cryptography", Martin E. Hellman, Scientific American, August 1979, Vol. 241, Number 2, pp. 146-157.
Unfortunately for narrow-bandwidth channels, however, the secure digital cryptographic signal with acceptable signal quality requires a wide bandwidth for proper signal transmission. A second secure communications approach utilizes frequency inversion of the analog signal to introduce security. This technique can remain within the bandwidth of a narrow band channel. The analog signal is not converted to digital representations, rather, the analog signal is mixed against a single frequency tone in a square-law mixer or balanced modulator and the lower sideband of the product of the tone and the analog signal is selected by a filter. The resultant signal is one in which the analog signal has the lowest frequency components and highest frequency components reversed and shifted in frequency.
The single tone frequency inversion scrambler is extremely easy to defeat. The eavesdropper need only to inject a single tone into a square law detector and adjust the tone frequency to be essentially identical to that used to initially invert the analog signal. Improvements to the frequency inversion scrambler have utilized multiple inversion tones sequenced over time in a pseudorandom fashion. Further improvements have utilized a combination of frequency inversion, time inversion, and time hopping segment permutation to make the narrow band scrambler more secure. (See U.S. Pat. No. 4,434,323). Each improvement, however, has increased the complexity and cost of the scrambling system and has further complicated the synchronization of the inversion hopping algorithm.